Sonic Underground

Argus, a captain of the Queen's guard, is captured by Sleet and Dingo before he can reach the secret rendezvous where he was to meet Queen Aleena. While listening to a scanned radio message, Sonic, Manic and Sonia overhear that a queen's guard has been captured and robotisized. The Hedgehogs then begin their pursuit to recapture captain Argus from Robotnik and get the information revealing the secret meeting place where they may be reunited with their beloved mother.

Sonic, Sonia and Manic return to their birthplace, Mobodoon, a beautiful, festive town. The homecoming is interrupted when Sleet and Dingo arrive to collect back taxes by order of Dr. Robotnik. Because Mobodoon has no form of currency to pay this bogus debt, Sleet and Dingo decide to raid the village and steal its most precious monuments, including the powerstone, which is the center of Mobodoon's vitality. It is up to the Hedgehogs to get the powerstone back and restore their former home to its previous blissful state.

Sonia travels to east Mobius to visit her friend Mindy at the LaTour estate. After being given a watch by her friend Mindy, Sonia discovers that these popular timepieces are really monitoring devices for the Robotnik Intelligence Agency and are mass produced by Mindy's father Mr. LaTour. Sonia, Sonic and Manic confront Mindy and LaTour with this discovery. Realizing the error of his ways, Mr. Latour ends production and decides to flee with his daughter. The Hedgehogs skillfully put the spy devices to their own use to help the Latour's escape.

Sleet and Dingo plan to collect the taxes owed to Robotnik from the Duke of Velcronia but keep it for themselves. To avoid the wrath of Robotnik, Sleet and Dingo infiltrate the Underground to corral Manic and friends to the commit the crime and take the blame. The scheme is moving along well for the evil duo until they are caught on tape by Max, Manic's friend, who stops the devious Sleet and Dingo enabling the Duke keep his money.

The sinister Robotnik blackmails Cyrus, an old friend of Sonic's, by holding his father hostage. Robotnik makes Cyrus gain the Hedgehogs trust and then has Cyrus convince them to save his father. Cyrus reluctantly informs Robotnik of the rescue plan so Robotnik can devise a trap for the Hedgehogs. However, once Cyrus sees the Hedgehogs helping the kids of Mobius, he realizes assisting in the capture of the Hedgehogs is a bad idea. Instead, Cyrus helps the Hedgehogs and in the process they destroy another of Robotnik's swat-bot factories.

On a trip to Port Mobius, Sonic, Manic and Sonia are told that ships are sinking due to the Mobian sea-beast. The legendary story becomes fishy when the Hedgehogs meet Captain Squeegee who informs them of a scam in which townspeople must rely upon Robotnik for protection from the sea monster, when in reality it is Robotnik who is sinking ships, not a monster. Sonic must overcome his fear of water and the monster to find the truth and thereby ending the fears of the sea side town.